1. technical field
This invention relates to a process for thermal cracking of a heavy hydrocarbon oil (hereinafter abbreviated as heavy oil) to obtain primarily light hydrocarbons (hereinafter abbreviated as light oils) which are liquid at room temperature. More particularly, the present invention relates to an improvement of a process comprising a step of thermal cracking in which a heavy oil is contacted with fine particles of a porous material fluidized with a steam-containing gas and a step of regeneration in which the coke deposited on said fine particles withdrawn from the thermal cracking step is removed while the fine particles are fluidized with a molecular oxygen-containing gas or a steam-containing gas, said fine particles being circulated between the both steps.
2. Prior art
Some of the present inventors have previously disclosed that thermal cracking can be practiced under good fluidized state of the bed and wherein with good efficiency when use is made of fine particles comprising particles having a weight average diameter of 0.04 to 0.12 mm and 5 to 50 wt. % of the particles have a diameter of 0.044 mm or less, and they named this process "Fluid Thermal Cracking" (Japanese Laid-Open Patent Publication No. 10587/1981).
They have also disclosed that when use is made of fine particles which have a pore volume of 0.1 to 1.5 m.sup.3 /g, a specific surface area of 50 to 1500 m.sup.2 /g and a weight average diameter of 0.025 to 0.25 mm and are thermally stable, thermal cracking can be practiced at still improved efficiency. They have found that absorption of liquid heavy oil by the pores of the porous material exhibits actions such as promotion of thermal cracking reaction or inhibition of formation of highly carbonaceous solid (hereinafter abbreviated as coke), and they called this "the capacitance effect" (see Japanese Laid-Open Patent Publication No. 18783/1982).
Further, they have disclosed a similar process, comprising a step of thermal cracking a heavy oil and gasification step (this is called a regeneration step in the present invention) for removing by gasification of the coke deposited on the fine particles of the porous material withdrawn from the thermal cracking step by contacting the fine particles with an oxygen-containing gas, while circulating said fine particles between the two steps. Further, an effective embodiment has been shown, in which the fluidized-beds formed in the both steps are arranged adjacent to both sides of a thermally conductive partition wall (see Japanese Laid-Open Patent Publication No. 158291/1982).
Whereas, concerning the process performing circulation of particles between the thermal cracking step and the regeneration step, there is a number of examples of practice and patents. In the fluid catalytic cracking process (FCC process) intended primarily for obtaining gasoline from light oil, each of the catalytic cracking step and the regeneration step is conducted according to the fluidized-bed reaction system, simultaneously with circulation of a relatively large amount of catalyst particles. On the other hand, in the fluid coking gasification process (hereinafter abbreviated as Flexicoking process), the coke particles formed are generally circulated between the thermal cracking step and the regeneration step comprising a combustion section and a gasification section, and further a heating section is added to the process, if desired. According to the Flexicoking process, the coke particles heated to high temperature at the combustion section are circulated, while being apportioned to the thermal cracking step and the gasification step, respectively, and the reaction heat necessary for the respective sections is supplied through the sensible heat thereof (see Japanese Laid-Open Patent Publication No. 108193/1982). Further, according to the Flexicoking process, there is a system in which the heat necessary for thermal cracking is supplied by circulation of the coke particles between the thermal cracking step and the gasification step, and the heat necessary for the gasification reaction is supplied by the heat of the coke particles circulated between the gasification step and the combustion step (see Japanese Laid-Open Patent Publication No. 76090/1982).
Each of such prior art is useful, but its practical usefulness would be further improved if the operational control could be done more easily.